In the Patent Literature 1, the switching interval is directly set to 24 hours or 1 hour. In the Non Patent Literature 1, the description is that “[0059] For each RRU in the area, any k antennas among all antennas are selected at a preset interval, and k paths of downlink baseband signals are transmitted to k antennas of each RRU, respectively”.? Moreover, since the actual traffic varies all the time, if the switching interval is set to a fixed value, the actual cost reduction effect may not be good all the time.